Selectable delay can be achieved by using RF switches to select different lengths of coaxial cables. The major problems with this approach are size and signal distortion. The propagation velocity of signals in cable is about 10.sup.8 meters per second; therefore, it takes about 100 meters of cable to delay signals by one microsecond. In addition, cables are typically dispersive (delay changes with frequency) and susceptible to reflections caused by impedance discontinuities. Significant signal distortion can result.
Selectable delay can also be effected with charge coupled devices which are time-sampled delay devices. The biggest difficult with the CCD's is that the sampling rate is relatively low. It is relatively difficult to make CCD's with sampling rates over 100 MHz (in silicon). In addition, although the sampling rate can be dynamically changed in a CCD (up to the limit of the chip), the delay is also changed when this occurs and distortion may also result. CCD's have not been widely used for selectable delay applications to our knowledge, and an additional reason for this is the difficulty in implementing such a function in a compact and user-friendly form. We are not aware of the existence of such a product, even in the lower frequency ranges where silicon CCD's are feasible.
Surface acoustic wave devices can be used to provide delay of signals with wide bandwidth. The two biggest difficulties in implementing selectable SAW delay lines are time-spurious signals and complexity of the tap-switching circuitry. Any spurious acoustic echoes or direct electromagnetic coupling of the input signal present in the SAW device output represent a signal distortion. These distortions are hard to control in wideband programmable SAW devices. Secondly, SAW devices are typically not constructed on semiconductor material, and therefore it is not possible to integrate the hundreds of active elements required to implement a multi-tap selectable delay line. Hybrid SAW selectable delay lines are quite complex and costly subsystems.
Signals can be delayed by inductor-capacitor (LC) networks designed as all-pass networks. The main difficulty with these delay lines is the dispersion that is inherent in the design. The dispersion can seriously distort pulse shapes in wideband applications. The problem of effectively changing the delay of such networks dynamically is also not easily solved.
Signals can be delayed utilizing A/D converters, high-speed digital memory, and D/A converters. The process of converting signals to digital form, storing and recalling them, and then converting back to analog is complex and requires a complex system to implement. The main difficulties with this approach are speed limitations presented by the A/D and D/A converters and other digital components, high system complexity required to implement it, and signal distortion resulting from the digitization process. DRFM's are complicated delay and signal processing systems used in electronic warfare (EW) applications which are quite large and very costly, mainly because of the complexity of utilizing the digital delay approach.
In the past, ACT devices have been operated with adjustable frequency generators and large power amplifiers. The frequency of operation of the SAW acoustic system had to be known before-hand for specific delivered RF powers and ambient temperature conditions or adjusted manually in situ. No fixed frequency RF oscillator/power amplifier could be expected to operate even the same type of ACT device because of frequency differences due to variations in manufacture or materials used in fabrication. Thermal control systems could be used to actively force the IC substrate to operate at a specific temperature (and consequently a constant frequency) but would be more complex and consume significantly greater amounts of electrical power. Alternately, multiple pick-up transducers could be used in the SAW path to hold the delay line phase response constant (and hence a constant number of wavelengths) in a negative feedback system using an electrical control system (phase detector controlling a variable phase shifter in the loop) and/or in combination with a thermal control system. Again, such a system is more complicated.